1. Field of the Invention:
This invention relates generally to throttle control systems for controlling the throttle opening by an electrical signal and, more particularly, to a throttle control system capable of maintaining a minimum function of operating with safety even if an accelerator sensor which determines a target throttle opening malfunctions.
2. Description of the Related Art:
In conventional throttle control systems for controlling a motor vehicle throttle valve by an electrical signal, the throttle opening of a motor vehicle is controlled on the basis of a signal representing the amount of accelerator operation from an accelerator sensor or the like and, therefore, there is a risk of a malfunction of the accelerator sensor for detecting the amount of accelerator pedal operation resulting in a serious vehicle accident. Various fail-safe techniques have therefore been proposed for the purpose of preventing uncontrollable speeding or the like of a motor vehicle when a malfunction of an accelerator sensor or the like occurs.
FIG. 5 is a block diagram of a motor vehicle throttle control system disclosed in Japanese Patent Laid-Open Publication No. 3-141841. In FIG. 5, a block 1 represents an accelerator pedal (hereinafter referred to simply as "accelerator"), and blocks 20 and 21 represent first and second accelerator sensors which detect the amount of depression (operation) of the accelerator 1 to output accelerator operation amount signals .alpha. 1 and .alpha. 2 corresponding to the amount of operation. An accelerator switch 22 outputs an on-off signal .beta. representing whether or not the accelerator 1 is depressed. The accelerator switch 22 is operated in response to a predetermined amount of operation (mentioned later) in the range of operation of the accelerator 1.
An engine revolution sensor 4 detects the number of revolution R of an engine 3. A throttle valve (hereinafter referred to simply as "throttle") 5 serves to control the intake air flow into the engine 3. A throttle actuator 6 drives the throttle 5 by an electrical signal. A throttle sensor 7 detects the actual opening .theta. of the throttle 5. A vehicle speed sensor 8 detects the speed V of the motor vehicle.
A control unit (control means) 10 formed of a microcomputer receives the accelerator operation amount signals .alpha. 1 and .alpha. 2, the output signal .beta., a signal representing the throttle opening .theta., the vehicle speed V and the engine revolutions R, and forms a throttle control signal .gamma. for the throttle actuator 6 according to a target opening of the throttle 5.
The control unit 10 has a throttle opening control section (not shown) for calculating a target opening of the throttle 5 on the basis of the accelerator operation amount signals .alpha..sub.1 and .alpha..sub.2, the vehicle speed V and the engine revolutions R and for making the throttle opening .theta. coincide with the target opening, and a malfunction detection section (not shown) for detecting a malfunction of the accelerator sensor 20 or 21 on the basis of the accelerator operation amount signals .alpha..sub.1 and .alpha..sub.2 and the output signal .beta. from the accelerator switch 22.
FIGS. 6A to 6C are characteristic diagrams showing the accelerator operation amount signals .alpha..sub.1 and .alpha..sub.2 and the output signal .beta. from the acceleration switch 22 with respect to the actual accelerator opening. FIG. 6A shows a characteristic of the accelerator operation amount signal .alpha..sub.1 from the accelerator sensor 20, FIG. 6B shows a characteristic of the accelerator operation amount signal .alpha..sub.2 from the accelerator sensor 21, and FIG. 6C shows a characteristic of the output signal .beta. from the accelerator switch 22.
In FIG. 6C, a value A on the abscissa represents a predetermined accelerator opening corresponding to a predetermined operation amount .alpha..sub.0 at which the accelerator switch 22 is turned on or off.
An example of a change in the signal shown in FIG. 6B indicates that a malfunction occurs in the accelerator sensor 21 in the range between accelerator openings B1 and B2.
The operation of the conventional throttle control system shown in FIG. 5 will be described below with reference to FIGS. 6A to 6C.
First, when a driver depresses the accelerator 1, the accelerator operation amount signals .alpha..sub.1 and .alpha..sub.2 corresponding to the amount of depression of the accelerator 1 are generated from the first and second accelerator sensors 20 and 21. The accelerator switch 22 is turned on if the driver depresses the accelerator 1 to the predetermined opening A, or it is off if the driver has not yet depressed the accelerator 1 to the predetermined accelerator opening A.
The control unit 10 obtains the average of the two accelerator operation amount signals .alpha..sub.1 and .alpha..sub.2, and calculates a target value of the throttle opening, i.e., a target opening, on the basis of information including the accelerator operation amount average, the engine revolutions R and the vehicle speed V. The control unit 10 then forms throttle control signal .gamma. such that the throttle opening .theta. detected by the throttle sensor 7 will coincide with the target opening, and outputs the throttle control signal .gamma. to the throttle actuator 6.
At this time, the control unit 10 recognizes, from the throttle opening .theta., whether the throttle 5 has been actually opened to the target opening, and controls the opening .theta. of the throttle 5 so that the target opening is reached.
Thus, the intention of the driver is detected as the amount of depression of the accelerator 1 through the accelerator sensors 20 and 21, transmitted to the control means 10 and used to control the throttle 5 through the throttle actuator 6, thus controlling the output of the engine 3.
If an abnormality of one of the accelerator sensors 20 and 21 occurs, a large voltage difference is caused between the accelerator operation amount signals .alpha..sub.1 and .alpha..sub.2.
The following is an example of the control operation in a case where the accelerator sensor 20 malfunctions. If the control unit 10 determines that the accelerator operation amount signal .alpha..sub.1 is greater than the predetermined value .alpha..sub.0 when the accelerator switch 22 is OFF or that the accelerator operation amount signal .alpha..sub.1 is smaller than the predetermined value .alpha..sub.0 when the accelerator switch 22 is ON, the control unit 10 then determines that there is an abnormality, i.e., the accelerator sensor 20 is malfunctioning, and thereafter changes its operation from the normal control to a control using only the accelerator operation amount signal .alpha..sub.2 from the accelerator sensor 21.
In such a case, i.e., in the case of a malfunction of one of the acceleration sensors 20 and 21, the other one functions to back up the malfunctioning sensor.
On the other hand, in the case of a malfunction of the accelerator switch 22, the malfunction is not detected and the ordinary throttle control based on the average of the accelerator operation amount signals .alpha..sub.1 and .alpha..sub.2 is continued, because the output signal .beta. from the accelerator switch 22 contributes only to the detection of malfunctions of the accelerator sensors 20 and 21 and does not relate directly to the throttle control.
In the above-described conventional method, however, malfunctions of the accelerator sensors 20 and 21 are not sufficiently considered and a malfunction cannot be detected under some condition.
For example, a case of a malfunction of the accelerator sensor 21, such as that shown in FIG. 6B, will be described below.
The output signal .beta. of the accelerator switch 22 is ON when the accelerator opening, which is the amount of depression of the accelerator 1, exceeds A %, and the output signal .beta. is OFF when the accelerator opening is smaller than this value. It is assumed here that a malfunction has occurred in the range of accelerator opening of B1 to B2% and the accelerator operation amount signal .alpha..sub.2 is largely different from .alpha..sub.1 and has a value .alpha..sub.2 =C [V].
The values A, B1 and B2 are in a relationship: A&lt;B1&lt;B2%, and the accelerator switch 22 is in the on state in the accelerator opening range of B1 to B2. The accelerator sensor 20 is in the normal state, the accelerator operation amount signal .alpha..sub.1 changes in the range of D1 to D2 [V] with respect to the accelerator opening range of B1 to B2.
In the case of such a malfunction of the accelerator sensor 21, a large difference is caused between the accelerator operation amount signals .alpha..sub.1 and .alpha..sub.2 in the accelerator opening range of B1 to B2. However, the control unit 10 cannot determine whether one of the accelerator sensors 20 and 21 malfunctions.
This is because, since the method of determining that one of two accelerator sensors malfunctions if it outputs an accelerator operation amount signal smaller than a predetermined value .alpha..sub.0 is used, a malfunction as in a direction such that the output increases as in the case of the accelerator operation amount signal .alpha..sub.2 (see FIG. 6B) cannot be determined if the output signal .beta. from the accelerator switch 22 is ON in the accelerator opening interval from B1 to B2%.
Also, when the accelerator switch 22 is off, both the accelerator sensors 20 and 21 output the accelerator operation amount signals .alpha..sub.1 and .alpha..sub.2 in the normal state and, therefore, it is also impossible of determine whether one of the accelerator sensors malfunctions.
Accordingly, the control unit 10 uses the average of the accelerator operation amount signals .alpha..sub.1 and .alpha..sub.2 as in the normal state and calculates a target throttle opening from the average of the value D1 to D2 of the accelerator operation amount signal .alpha..sub.1 and the value C of the accelerator operation amount signal .alpha. 2 in the accelerator opening range of B1 to B2. The throttle 5 is thereby opened abruptly. Such a control failure may lead to excessive speeding of the vehicle in the worst case.
With respect to such a malfunction, processing of selecting the smaller one of the accelerator operation amount signals .alpha..sub.1 and .alpha..sub.2 may be performed to prevent abrupt opening of the throttle 5. In the case of using such processing, however, if a malfunction occurs as in a direction such that the accelerator operation amount signal .alpha..sub.1 or .alpha..sub.2 decreases, there is a possibility of the throttle 5 being fully closed, i.e., a state of the vehicle being disable from traveling.
In the conventional throttle control system as described above, malfunctions of each of the accelerator sensors 20 and 21 cannot always be detected under all possible conditions and, moreover, a malfunction of the accelerator switch 22 cannot be detected. It is therefore impossible to maintain a minimum function of operating with safety.